1. J. Wheeler, chapter inQuantum Theory and Gravitation (Proceedings, symposium held at Loyola University, New Orleans, May 23–26, 1979) (Academic, New York, 1980).
2. D. Finkelstein,Int. J. Theor. Phys. 27 (4), 473 (1985).
3. A. V. Evako,Int. J. Theor. Phys. 33 (7), 1553 (1994).
4. L. Bombelli, J. Lee, D. Meyer, and R. Sorkin, preprint IA SSNS-HEP-87/23, Institute of Advanced Study, Princeton, 1987.
5. H. S. Snyder,Phys. Rev. 71, 38 (1947).